1. Field of the Invention
This invention relates generally to an adaptive control variable gear ratio (VGR) steering system and, more particularly, to an adaptive control VGR steering system that changes the gear ratio between a vehicle hand-wheel angle and the road wheel angle based on vehicle speed and one or more of hand-wheel angle, driver driving style and skill, and driver attentiveness.
2. Discussion of the Related Art
The steering gear ratio of a vehicle is a proportional factor between the steering wheel angle and the road wheel angle. Conventional steering systems have a fixed steering gear ratio, where the steering gear ratio remains substantially constant except for minor variations due to the suspension geometry of the vehicle. In order to improve handling for all driving conditions, variable gear ratio (VGR) steering systems have been provided. For VGR steering systems, the gear ratio varies with vehicle speed so that the number of steering wheel turns to make a vehicle turn is low at low vehicle speeds and high for high vehicle speeds. In other words, the gear ratio between the vehicle hand-wheel angle and the road wheel angle increases as the vehicle speed increases so that turning is easier at lower speeds and steering sensitivity is suppressed at higher speeds where stability may be a factor. The VGR steering system can be implemented by adding an angle to or subtracting an angle from the driver's steering input using a gear differential between the steering wheel and the steering gear. A planetary gear set or a harmonic drive can be used to create such a differential.
Known steering systems focus on the steering response of on-center handling, where the steering wheel angle is relatively small and the tires are in their linear region. As the steering wheel angle increases and the tires enter their non-linear region, the vehicle steering sensitivity decreases, requiring the driver to steer more to maintain a desired steering-to-yaw-rate gain.
Practically, the design of known steering systems is a compromise to meet the needs of all types of drivers with one single speed-VGR curve. Nonetheless, many customers, especially sporty drivers, expect electric aids to enhance their driving experience, even in situations that an average driver would never encounter. Also, automotive enthusiast magazines show that vehicle evaluation often focuses on high-performance driving. The challenge for the design is to create a system that satisfies average drivers as well as motor sport enthusiasts.
In addition, a drivers psychological and physiological conditions also affect his or her ability to control a vehicle safely. Although the detection of a driver's psychological condition, such as emotion and stress, is still immature, various systems have been developed to detect a drivers physiological conditions. Vehicle control systems, such as VGR steering systems, should be able to work with those driver-condition detection systems and adapt themselves to driver conditions to further enhance roadway safety.